Title:
The impact of baryons on the direct detection of dark matter

Abstract: The spatial and velocity distributions of dark matter particles in the Milky
Way Halo affect the signals expected to be observed in searches for dark
matter. Results from direct detection experiments are often analyzed assuming a
simple isothermal distribution of dark matter, the Standard Halo Model (SHM).
Yet there has been skepticism regarding the validity of this simple model due
to the complicated gravitational collapse and merger history of actual
galaxies. In this paper we compare the SHM to the results of cosmological
hydrodynamical simulations of galaxy formation to investigate whether or not
the SHM is a good representation of the true WIMP distribution in the analysis
of direct detection data. We examine two Milky Way-like galaxies from the
MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic
physics included. The inclusion of baryons drives the shape of the DM halo to
become more spherical and makes the velocity distribution of dark matter
particles less anisotropic especially at large heliocentric velocities, thereby
making the SHM a better fit. We also note that we do not find a significant
disk-like rotating dark matter component in either of the two galaxy halos with
baryons that we examine, suggesting that dark disks are not a generic
prediction of cosmological hydrodynamical simulations. We conclude that in the
Solar neighborhood, the SHM is in fact a good approximation to the true dark
matter distribution in these cosmological simulations (with baryons) which are
reasonable representations of the Milky Way, and hence can also be used for the
purpose of dark matter direct detection calculations.